Method of inhibiting formation of black spots on tinned copper



Patented Dec. 14, 1943 METHOD OF INHIBITING FORMATION BIJACK SPOTS ON TINNED COPPER Herbert G. Jennison, deceased, late of Fairfield,

001111., by Helen E. Jennison, executrix, Fairfield, Conn., and John 0. Bradley, Waterbury, Comm, assignors to The American Brass Company, Waterbury, Conn., a corporation of Connecticut No Drawing. Application January 16, 1941,

, Serial No. 374,701

8 Claims.

This invention relates to a method of inhibiting formation of black spots on tinned copper, and this application is a continuation in part of our prior application, Serial No. 189,786, filed February 10, 1938.

For years the dairy. machinery industry has used tinned copper for making milk vats, coolers, etc. Tinned copper, that .is copper coated with a thin layer of tin, is liked for these articles because it is of comparatively low cost and can be handled and fabricated more easily than the substitutes, such as enamel ware, stainless steel, rich nickel alloys, etc. There is, however, one

particularly serious difliculty with tinned copper which is known as black spots or tin pest. It is a serious problem in caring for dairy apparatus. This condition is not due to corrosion of the base metal but rather to an allotropic change in the tin which results in disintegration of the tin coating. Our method prevents or greatly hinders this allotropic change.

It is therefore an. object of the present invention to prevent or delay the formation of these black spots, and we have found that this may be done by incorporating a small amount of one ormore of certain other metals with the tin. We have found that when a small amount of one or more of the metals of the group consisting of manganese and chromium are deposited with the tin, either simultaneously or successively, the formation of these black spots is material- 1y delayed. It is thought that there is a real alloy formed of tin and other metal or metals at the surface, and that this alloy inhibits the allotropic change in the tin.

As an example when copper was plated by electrodeposition with tin and manganese, manganese sulphate and ammonium sulphate were used. The manganese sulphate was present inthe quantity of about 37.5 grams in 125 cc. of water. To this was added 25 cc. of the standard sodium stannate electrolyte of substantially the following solution;

Grams per liter Sodium stannate 32 Sodium acetate .6 Sodium perborate .3 Rosin .01

While this gave good results other solutions mightbe used. A precipitate which is formed was dissolved with a minimum amount of sula black spongy deposit which'had to be cleaned ofi every two or three minutes, but underneath of commercially pure tin is first put on to the copper and then the tin-manganese coat deposited. Or wecan put on the metals separately and in succession, first the layer of tin, then the other metal or metals. Or if desired, we can add to the second coat a third coat of tin or a-combination of the tin and the manganese.-

As indicated above we have found that chromium when plated in combination with tin in the same manner also prevents or greatly hinders the formation of black spots.

When the copper was plated with chromium the plating was accomplished by depositing al. ternate layers of chromium and tin, then the cop-- per was heated for an hour in boiling water. For the chromium plating solution any one of a number of good solutions could be used. As an example we used a solution substantially as follows:

Grams per liter Chromic acid figured as CrO: 40 Sulphate figured as S04 4 The plated sheet of copper when tested showed improvement over a pure tin" coating in resistance to formation of black'spots. Analysis showed chromium and tin present with the tin greatly v predominating.

We are not confined'to the use of pure metals. The point is that when tin is deposited on the copper in combinationlwith certain other metals the black spot formation is materially hindered. The resultant coating'is essentially a tin coating V with a small amount of theother metal or metals deposition of the metals from a single electroplating bath, but as suggested above we have also accomplished the desired end by successive deposits from different baths.

We do not wish to fuse the tin coating either in depositing or after it has been deposited. Fusing a tin coating on copper definitely thins the tin coat by making the tin diffuse into the copper and thus really result in a bronze coating of mostly copper instead of a tin coating or coating mostly tin, In other words subsequent fusing of the tin coating is undesirable in our method for inhibiting black spots as our experiments have indicated it tends to reduce the resistance of the coating to this disease.

' Having thus set forth the nature of our invention, what we claim is;

1. A method of inhibiting formation of black spots on tinned copper by contact with liquids, food and beverage products which consists in electrodeposi-ting on the copper a coating of tin and at least oneof the metals of the group consisting of manganese and chromium inamount sufficient to inhibit black spot formation and with the resultant coating essentially tin and of at least about 90 per cent of this metal.

2. A method of inhibiting formation of black spots on tinned copperwhich consists in 'electrodepositing on the copper a coating essentially tin and metal of the group consisting of manganese and chromium in amount suflicient to inhibit black spot formation and not over about per cent of the resultant coating.

3. A method of inhibiting formation of black spots on tinned copper which consists in electrodepositing on the copper surface a coating consisting essentially of tin and an amount of metal of the group consisting of manganese and chromium sufficient to inhibit black spot formation,

and with the resultant coating essentially tin and of at least about 90 per' cent of this metal.

4. A method of inhibiting formation of black,

sively electrodepositing on'thecopper a coating essentially of tin, and other metal from the group consisting of manganese and chromium in amount suflicient to inhibit black spot formation with the resultant coating being at least about 90 per cent tin.

6. A method of inhibiting formation of black spotson tinned copper which consists in the electrical deposition on the copper of a layer of metal by codeposition of tin and metal of the group consisting of manganese and chromium in amount suflicient to inhibit black spot formation from a single electroplating bath, and with the resultant coating essentially tin and of at least about 90 per cent of this metal.

'7. An article of manufacture for use in handling food and beverage products, including fabricated vessels,. containers or structures made at' least partly of copper and an electrodeposited coating on the copper consisting essentially of tin and metal of the group consisting of manganese and chromium in amount sufficient to inhibit black spot formation but not over about 10 percent of the resultant coating, said coating being on' that surface to be used in contact with the'food and beverage products whereby the tendency to formation of black spots on-the coated copper is inhibited.

8. An article'of manufacture for use in handling food and beverage products, including fabricated vessels, containers or structures made at least partly of copper and an electrodeposite'd alloy coating on the copper consisting essentially of-tin and a metal of the group consisting of inhibit black spot formation and with the tin not lessthan about 90 per cent, said coating being on that surface to be used in contact with the food and beverage products whereby the tendency 40 to formation of black spots on the coated copper is inhibited.

HELEN E. JENNISON,

Executrizc of Herbert C. Jenm'son, Deceased.

' JOHN C. BRADLEY.

manganese and chromium in amount sufficient to- CERTIFICATE okcomcwrok. P'gtent No. 2, 56,616. I ecember 1b,, 191 o HERBERT'C. mmuson, DECEASED, ET AL' It 15 hereby certified that error appears in the printed specification of the above numbered patent requiring correction asfollows: Page 1, second column, l ine B-k9, for "code deposition read --codepos1t1on-; and that the said Letters Patent should be read with this correction therein that the samemw'confon'n to the record of the case in the Patent Office. Signed and. sealed this 29th day pf February, A. o. 19%.

Leslie Frazer (3 81). Acting commissiorler of Patents. 

